The present invention relates to a display apparatus, such as a liquid crystal display apparatus (for example, a projection display, a view finder, a head mount display), a plasma display apparatus, a digital mirror display apparatus, an electroluminescent display apparatus, and a field emission display apparatus.
A popular technique of displaying images on these several types of display apparatus is to convert analog video signals into digital video signals and apply the digital video signals to pixels, with division of one TV field into several sequential subfields having different weights.
These types of display apparatus have such a subfield structure, even under different drive sequences, thus causing pseudo contour effects in moving-picture displaying.
One TV field has, for example, several 8-bit subfields with the ratio of duration 1 (=20): 2 (=21): 4 (=22): 8 (=23): 16 (=24): 32 (=25): 64 (=26): 128 (=27).
Combination of these subfields achieves 256 gradation levels (0 to 255). Pseudo contour effects in moving pictures could be caused by difference in emission timing at these several subfields. The quality of moving-picture images is lowered due to pseudo contour effects as the pictures move fast because the difference in emission timing appears as spatial difference.
Illustrated in FIG. 1A is occurrence of pseudo contour effects. There are 8 successive subfields SF1 to SF8 at gradation levels 127 and 128 for adjacent two pixels. An image is displayed from the subfields SF1 to SF8. Dot areas indicate off(black)-display whereas no-dot areas on(white)-display. The higher the gradation level, the doser to white whereas the lower the gradation level, the doser to black. Signs S1, S2 and S3 indicate the height or position of viewer's line of vision.
At the position S2, light Y1 passing through the subfields SF1 to SF7 of white (pixel on) and the subfield SF8 of black (pixel off) enters viewer's eyes correctly at the level 127.
When the viewer's line of vision is shifted from the middle position S2 to the upper position S1, light Y2 passing through the subfields SF1 to SF8 of black (almost no light) enters viewer's eyes, thus gradation being lowered from the level 127 to the level 0 (pure black). This shift of line of vision generates pseudo contours of black level which cause pseudo contour effects.
On the contrary, when the viewer's line of vision is shifted from the middle position S2 to the lower position S3, light Y3 passing through the subfields SF1 to SF8 of white enters viewer's eyes, thus gradation being raised from the level 127 to the level 255 (pure white). This shift of line of vision generates pseudo contours white level which also cause pseudo contour effects.
There are several proposals for reducing moving-picture pseudo contour effects, for example, disclosed in non-patent literature (1) “DLP Projection System” display and Imaging 2001, Vol. 9, pp 79 to 86; non-patent literature (2) Nikkei Electronics Apr. 10, 1999 (No. 753); patent literature (1) U. S. Pat. No. 6,151,011; and patent literature (2) Japanese Un-examined Patent Publication No. 2001-343950.
The non-patent literature (1) discloses division of subfields having long display periods into several subfields having short display periods and rearrangements to the subfields of short display periods in one TV field. The subfield division and rearrangements minimizes displacement of light emitted from a display device in the direction of time when a subfield that is on is switched to the next subfield, thus pseudo contour effects being hardly observed.
The patent literature (1) also discloses division of subfields having long display periods into several subfields having short display periods in a liquid crystal display apparatus.
Illustrated in FIG. 1B is reduction of pseudo contour effects in the patent literature (1). There are 19 successive subfields SF1 to SF19 at gradation levels 126 to 130. Dot areas indicate off(black)-display whereas no-dot areas on(white)-display. The number in each no-dot area indicates the level of brightness. For example, the subfields SF8 to SF11 are off whereas the subfield SF12 is on at the gradation level 128.
The shift of viewer's line of vision from the position S2 to S1 or S3 does not cause pseudo contour effects because light Y1 to Y3 enter viewer's eyes at the same gradation level 127.
The non-patent literature (2) discloses CLEAR (Hi-Contrast and Low Energy Address and Reduction of False Contour Sequence) that controls emission period in accordance with brightness in a plasma display apparatus for reduction of false contour sequence or pseudo contour effects. The patent literature (2) also discloses control of emission period in accordance with brightness for reduction of pseudo contour effects.
The known techniques disclosed in the above literatures, however, have the following drawbacks, as illustrated in FIG. 1C.
Suppose that, instead of the subfield SF12 which is on at the gradation level 128 as shown in FIG. 1B, when the subfield SF 17 apart from the subfield SF12 is turned on at the gradation level 128 as shown in FIG. 1C, based on the disclosures in the patent literature (1) and non-patent literature (1).
At the position S2, light Y1 enters viewer's eyes correctly at the gradation level 127. The shift of the viewer's line of vision from the position S2 to S1 (Y1 to Y2) also achieves the gradation level 127. In contrast, the shift from the position S2 to S3 (Y1 to Y3) raises the gradation from the level 127 to the level 143, change in gradation by 16 levels (=143−127), which causes pseudo contour effects at the boundary between the positions S2 and S3.
The non-patent literature (2) employs 24 subfields per 2 fields for the maximum 24 gradation levels, thus requiring signal processing, such as, dither and error diffusion, for full gradation displays.
Moreover, the patent literature (2) requires additional controlling circuitry for converting bit plane into subfield per pixel in accordance with brightness because no look-up table of each subfield to be accessed and the corresponding bit plane to be displayed being provided, thus being complex in circuit configuration.